1. Technical Field
This disclosure relates generally to an improved structure of a spark plug which may be used in internal combustion engines for automotive vehicles and is designed to ensure a desired degree of ignitability of fuel.
2. Background Art
Spark plugs for automotive internal combustion engines are known which have a center electrode extending in an axial direction of the spark plug with a top end facing a ground electrode to form a spark gap therebetween. This type of spark plugs work to develop sparks across the gap to ignite an air-fuel mixture in a combustion chamber of the engine.
Generally, rotating streams of the mixture such as a swirl or a tumble are created within the combustion chamber. Such streams at least partially passes through the spark gap, thereby ensuring the ignitability of the mixture.
The part of the ground electrode welded to an end of a housing (usually called a metal shell) of the spark plug may be located upstream of the spark plug in terms of the streams of the mixture depending upon an angular orientation of the spark plug screwed into the head of the engine. This causes the streams of the mixture to be blocked partially by the ground electrode within the combustion chamber, so that they may stall near the spark gap, thus resulting in a decrease in ignitability of the mixture. The problem of a variation in ignitability of the mixture is, therefore, encountered depending upon the angular orientation of the spark plug installed in the engine. Recent years, a lot of lean-burn internal combustion engines have been used. Such type of engine may experience the instability of burning of fuel depending upon the angular orientation of the spark plug installed in the engine.
It is usually difficult to set the angular orientation or position of the ground electrode of the spark plug in a direction of rotation thereof. This is because it depends upon the geometry of a thread on the meta shell screwed into the engine head or the degree with which the thread is tightened into the engine head.
In order to alleviate the above problem, Japanese Patent First Publication No. 9-148045 teaches a spark plug designed to have the ground electrode with a hole through which the streams of the mixture is admitted to pass or joining of the ground electrode to the metal shell using a plurality of thin plates.
The hole in the ground electrode, however, results in a decrease in mechanical strength of the metal shell. An increase in thickness of the ground electrode in order to eliminate such a problem will also result in an increase in obstruction to the streams of the mixture toward the spark gap.
The use of the thin plates to join the ground electrode to the metal shell results in a complex structure of the spark plug and an increase in production costs thereof.